1. Field of the Invention
The present invention relates to an electromagnetic wave measuring apparatus and a method for evaluating and measuring radiation directivity of an electromagnetic wave using the same.
2. Description of the Related Art
Generally, an electromagnetic wave radiated from an antenna or an electromagnetic wave radiated from a device itself are evaluated for its energy and directivity in a method in which a device to be evaluated is placed using a large anechoic chamber and an electromagnetic wave thereof is received and observed at a sufficiently distant position. The reason for using the RF anechoic chamber is to prevent an electromagnetic wave from entering from the outside and to eliminate an effect of a reflected wave upon the measurement. The reason for performing the measurement at a sufficiently distant position is to measure the radiated electromagnetic wave in the far field and to reduce an error caused by a changed sensitivity due to an effect of the device to be evaluated upon a receiving antenna.
This method, however, involves very large evaluating equipment (for example, 10 m.times.10 m.times.10 m in size). Thus, a near field measuring method (Tasuku Teshirogi: "Measurement in Neighborhood Field for Antenna", Shingakusi, vol.62, no.10, pp.1145-1153, October 1979) has been developed as a method for evaluating an antenna. The near field measuring method includes: (a) plane scanning; (b) cylindrical plane scanning; and (c) spherical scanning. The methods (b) and (c) imply the extreme difficulty of accurately measuring the perimeter of a device to be evaluated and involve a large amount of operation required for correction of a probe antenna and for conversion from a near field to a far field, so that these methods have not been brought into practice widely. Thus, the method (a) has been used generally.
However, the above-mentioned conventional, (a) plane scanning method, has a disadvantage that it can be adapted only to an antenna with a relatively narrow directivity such as an aperture antenna. The cause thereof is that the method performs conversion from the near field to the far field on the assumption that a source of an electromagnetic wave is planar.